The purpose of the proposed work is to identify and characterize receptors for the steroid hormone, 1,25-dihydroxyvitamin D3 (calcitriol) in the pituitary and brain and to determine the functional role of these receptors. The specific objectives for this 1-year period are a) to further characterize the high-affinity nuclear receptors we have found for calcitriol in the bovine pituitary, b) to determine the role of physiological state and cell type on calcitriol binding by the use of pituitary tissues from vitamin D-deficient and from lactating rats as well as from cultures of pituitary cell lines which secrete specific hormones, c) to search for calcitriol receptors in certain brain areas, including cerebral cortex, cerebellum, and hypothalamus, d) to begin to investigate the role of the calcitriol receptors by determining the effect of calcitriol on prolactin and growth hormone secretion in GH3 cells, e) to investigate the role of neurotransmitters in modulating calcitriol-receptor binding as well as any responses found, and f) to investigate the effect of calcitriol on neurotransmitter-receptor binding.